I contacted DigitalOcean about this and this is the reply I got. I think DigitalOcean is intentionally blocking users from installing/ testing Windows on their platform:

"Hello, Thank you for contacting DigitalOcean!
We do not provide windows OS on our end. Our Infrastructure supports Linux OS which we provide only. We do not encourage users to install Windows on our Platform as it will render your Droplet useless. If you need windows, please seek another IAAS provider if which can serve your hosting needs. Please feel free to reach out to us via this ticket if you have further queries or concerns, we will be around to help you out!
Kind Regards,
Sandesh
Platform Support Specialist
DigitalOcean "